The present invention relates to raster-type displays, such as televisions and computer displays, and more particularly to a system for detecting odd-field type blanking in 1:1 interlace conditions, and correcting the vertical blanking so that full lines of information appear at the top and bottom of the display.
In raster-type displays, the user may elect to select from a plurality of scan rates and interlace conditions, depending on the particular application. Most television systems are interlaced to avoid a visible flicker in the display, usually in a 2:1 ratio. Thus, the 2:1 interlace ratio means that two entire display fields must be developed to generate one frame, or complete image. For example, the most common rate of 525/60 means that there are 525 scanning lines per television frame with one field developed in 1/60 second. Normally, the interlace ratio is 2:1, that is, a frame will be composed of two fields so that each field comprises one-half the number of scanning lines per frame, in this example, 262.5.
Television operation in 1:1 interlace, or non-interlace as it is sometimes referred to, is becoming more prevalent, particularly in the area of computer displays. The change from 2:1 to 1:1 interlace is made by making the count of scan lines in two fields either an even or odd number. An undesirable feature of random switching between the two is that the first and last scan lines in the non-interlaced (1:1 interlace) field may be either whole or half, depending on the logic condition at the instant of selection. The instant that the switch change impacts on the logic determines which condition comes up, either the odd field or the even field. Even though the same field is repeated sequentially, it can be either odd or even, with the odd field having a half-line at the top and bottom of the display. This is considered an undesirable condition in 1:1 interlace, and must be corrected, if the condition occurs.